Second order differential highpass filter

ABSTRACT

A second-order differential highpass filter constructed according to the present invention includes a difference amplifier and a feedback processing circuit. The difference amplifier includes an operational amplifier OP 1 , and four resistors R 3 , R 4 , R 5  and R 6 , wherein R 4  /R 3  =R 6  /R 5 . An input voltage V 1  is fed to the inverting terminal (-) of the operational amplifier OP 1  via the resistor R 5 . Another input voltage V 2  is fed to the noninverting terminal (+) of the operational amplifier OP 1  via the resistor R 3 . The output of the operational amplifier OP 1  is fed back to the inverting terminal (-) of the operational amplifier OP 1  via the resistor R 6 . The feedback processing circuit includes an operational amplifier OP 2 , two resistors R 1  and R 2 , and two serial capacitors C 2  and C 1 . The output of the operational amplifier OP 2  is fed back to the inverting terminal (-) of the operational amplifier OP 2  via the two serial capacitors C 1  and C 2 , and is connected to the noninverting terminal of the operational amplifier OP 1  via the resistor R 4 , and the noninverting (+) terminal of the operational amplifier OP 2  is grounded. The inverting terminal of the operational amplifier OP 2  is connected to the output terminal of the operational amplifier OP 1  via the resistor R 1 . The two serial capacitors C 1  and C 2  are connected therebetween to one terminal of the resistor R 2  of which another terminal is grounded.

FIELD OF THE INVENTION

The present invention is related to a differential highpass filter, and in particular to a second-order differential highpass filter which is able to remove low-frequency signal with attenuation of -40 dB/decade and has a differential gain as well.

BACKGROUND OF THE INVENTION

Filters are almost inevitably required for electrical signals processing. In human body, there are many diagnosis-valued signals which are presented as differential signal, e.g. electrocardiogram, and electroencephalogram, etc.. However, upon recording these physiological signals the measured signals are often corrupted by low-frequency interference. The interferences are caused by the respiration and motion of the subject as well as the differential dc voltage due to the polarization of the electrodes. As a result, the measured signals are degraded or the physiological interpretations are disrupted. Thus, suppressing the low-frequency interferences become important and is the first step in the preprocessing of physiological signals.

In the literature, there are many excellent digital signal processing techniques to remove the low-frequency interference. When the magnitude of the low-frequency interference is large, the resolution of signals in analog-to-digit conversion is limited, and the converted data have not enough precision for physiological interpretations. Hence, the interference should be attenuated by an analog highpass filter before performing analog-to-digit conversion. A novel idea to solve the precision-limited problem is to consider an additional highpass filtering function in a preamplifier. Several circuits which feed the output signals back to the input stage via the integrating network have been developed to realize this idea. However, the integrating network only forms a first-order highpass filter in the preamplifier. The first-order highpass filter filter having an attenuation of -20 dB/decade is not good enough to suppress the high-intensity low-frequency interference. In order to obtain a desired better suppression of unwanted interference, a new circuit is presented in this invention for the realization of a second-order differential highpass filter.

SUMMARY OF THE INVENTION

A second-order differential highpass filter constructed according to the present invention comprises a difference amplifier and a feedback processing circuit. The difference amplifier comprises an operational amplifier OP₁, and four resistors R₃, R₄, R₅ and R₆, wherein R₄ /R₃ =R₆ /R₅. An input voltage V₁ is fed to the inverting terminal (-) of the operational amplifier OP₁ via the resistor R5. Another input voltage V₂ is fed to the noninverting terminal (+) of the operational amplifier OP₁ via the resistor R₃. The output of the operational amplifier OP₁ is fed back to the inverting terminal (-) of the operational amplifier OP₁ via the resistor R₆. The feedback processing circuit comprises an operational amplifier OP₂, two resistors R₁ and R2, and two serial capacitors C₂ and C₁. The output of the operational amplifier OP₂ is fed back to the inverting terminal (-) of the operational amplifier OP₂ via the two serial capacitors C₁ and C₂, and is connected to the noninverting terminal of the operational amplifier OP₁ via the resistor R₄, and the noninverting (+) terminal of the operational amplifier OP2 is grounded. The inverting terminal of the operational amplifier OP2 is connected to the output terminal of the operational amplifier OP₁ via the resistor R₁. The two serial capacitors C₁ and C₂ are connected therebetween to one terminal of the resistor R₂ of which another terminal is grounded.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of the preferred embodiments of the invention with reference to the accompanying drawings wherein like numerals represent like elements and in which:

FIG. 1 shows a block diagram of a second-order differential highpass filter constructed according to the present invention;

FIG. 2 is a frequency vs. volt plot which shows a simulation of frequency response of the second-order differential highpass filter shown in FIG. 1, wherein the differential input V₂ -V₁ =1V and -3 dB frequency F₀ =100 Hz;

FIG. 3 is a frequency vs. dB plot which shows a simulation of frequency response of the second-order differential highpass filter shown in FIG. 1, wherein the low-frequency suppression is -40 dB/decade;

FIG. 4 shows a block diagram of a second-order differential highpass filter of the present invention realized with an instrumentation amplifier;

FIG. 5 shows test results of the second-order differential highpass filter shown in FIG. 4, wherein signal (a) is one of the input V₁ =100 sin 2π500t mV; signal (b) is the other input V₂ =100 sin 2π30t mV; and signal (c) is output voltage which can be represented by -100 sin 2π500t mV; and

FIG. 6 is a frequency vs. volt plot which shows frequency response of the second-order differential highpass filter shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A second-order differential highpass filter is disclosed in the present invention, which comprises a difference amplifier (1) and a feedback processing circuit (2) as shown in FIG. 1.

The difference amplifier (1) includes an operational amplifier OP₁ and four resistors R₃, R₄, R₅, and R₆, wherein R₄ /R₃ =R₆ /R₅. The resistors R₅ and R₃ are connected to the inverting terminal (-) and the noninverting terminal (+) of the operational amplifier OP₁, respectively, so that an input voltage V₁ is able to be fed to the inverting terminal (-) of the operational amplifier OP₁ via the resistor R₅ and another input voltage V₂ is able to be fed to the noninverting terminal (+) of the operational amplifier OP₁ via the resistor R₃. The output terminal of the operational amplifier OP₁ is connected to the inverting terminal of the operational amplifier OP₁ with the resistor R₆ to form a feedback configuration. An output voltage V₀ resulting from the differential signal V₂ -V₁ is thus obtained from the output terminal of the operational amplifier OP₁.

The feedback processing circuit (2) consists essentially of an operational amplifiers OP₂, two resistors R₁ and R₂, and two capacitors C₁ and C₂, wherein the two capacitors C₁ and C₂ are connected to each other with a conductive wire in series. The output terminal of the operational amplifier OP2 is connected to the inverting terminal (-) of the operational amplifier OP2 with the the two capacitors C₂ and C₁ to form a feedback configuration as well as to the noninverting terminal of the operational amplifier OP₁ via the resistor R₄, and the noninverting terminal (+) of the operational amplifier OP2 is grounded. One terminal of the resistor R2 is connected to the conductive wire connecting the two serial capacitors C₁ and C₂, and another terminal thereof is grounded. The inverting terminal of the operational amplifier OP₂ is connected to the output terminal of the operational amplifier OP₁ via the resistor R₁.

A. Transfer Function

The structure of the proposed second-order differential highpass filter as illustrated in FIG. 1 comprises two operational amplifiers, two capacitors, and six resistors. After some manipulation, the transfer function F(S) is given by ##EQU1## where Q is quality factor;

ω₀ is -3 dB frequency;

K is differential gain;

V₀ is the output voltage of the second-order differential highpass filter;

V₁ and V₂ are the input voltage;

S is parameter of Laplace;

R₁ and R2 are resistors; and

C₁ and C2 are capacitors

It is apparent that the proposed filter shown in FIG. 1 is a second-order highpass filter for differential signal with a voltage gain. From the equations (3) and (4), the -3 dB frequency and quality factor of the filter are given by ##EQU2## B. Design Procedure

Given the desired specification of ω₀ and Q, the passive components can be determined by the following steps:

step 1: choosing suitable capacitance for C₁ and C₂ ;

step 2: calculating the resistance of R₁ by the equation: ##EQU3## step 3: calculating resistance of R₂ as follows: ##EQU4## The design procedure is very simple and easy to be implemented. It is not necessary to solve difficult quadratic equations, which usually results in complex-value or negative solutions.

C. Implementation by Instrumentation Amplifier

As shown in FIG. 4, the difference amplifier (1) shown in FIG. 1 can be realized with an instrumentation amplifier AD620 (Analog Devices, U.S.). In addition to the function of second-order highpass filtering, the proposed filter shown in FIG. 4 preserves the main properties of the instrumentation amplifier, e.g. low noises, high input impedance, high CMRR, etc.

The technique of this invention, a second-order differential highpass filter, can be made into a monolithic integrated circuit which functions as a preamplifier with a second-order highpass filter.

D. Results of Computer Simulation

To demonstrate the feasibility of this invention, a second-order differential highpass filter, computer simulation with Pspice software was carried out and described as follows:

Given the desired quality factor Q=0.707, -3 dB frequency F₀ =100 Hz, capacitors C₁ =148 nF and C₁ =123 nF, the two resistance of R₁ and R₂ can be obtained according to the step 2 and step 3 of the Design Procedure, which are R₁ =16.75 5 KΩ, R₂ =8.31 KΩ. Assuming the input voltage V₁ =1V(ac), V₂ =2V(ac), and the resistors R₃ =R₄ =R₅ =R₆ =10 KΩ, the frequency response of the proposed second-order differential highpass filter depicted in FIG. 1 was obtained and shown in FIG. 2 and FIG. 3 by volt and decibel (dB) respectively.

The units of horizontal and vertical axes in FIG. 2 are Hertz (Hz) and volt respectively. The output voltage is less than 0.707V if the frequency is less than 100 Hz, and the output voltage decreases as the frequency decreases. When the frequency is higher than 200 Hz, the output voltage is 1V nearly. It is apparent that FIG. 2 is a typical frequency response of a highpass filter.

The units of horizontal and vertical axes in FIG. 3 are Hertz (Hz) and decibel (dB), respectively. The performance of low-frequency suppression is demonstrated by the attenuation of -40 dB/decade. The results of the simulation verify the theoretical analysis and demonstrate the applicability of the second-order differential highpass filter proposed in FIG. 1.

E. Results of Practical Tests

The circuit depicted in FIG. 1 was realized with an instrumentation amplifier AD620 as shown in FIG. 4. The values of the passive elements were the same as those used in the above simulation (quality factor Q=0.707, -3 dB frequency F₀, =100 Hz.

FIG. 5 shows test results of the second-order differential highpass filter shown in FIG. 4, wherein signal (a) is one of the input V₁ =100 sin 2π500t mV; signal (b) is the other input V₂ =100 sin 2π500 t mV; and signal (c) is output voltage which can be represented by -100 sin 27c500t mV with a little sinusoid of 30 Hz. It can be seen from FIG. 5 that the sinusoid signal (signal (b)) with the frequency of 30 Hz has been substantially removed and the phase of the sinusoid signal (signal (a)) has been inverted at the output terminal of the filter (signal (c). The time scale is 10 ms/div and the amplitude scale is 100 mV/div.

FIG. 6 is the frequency response of the proposed second-order differential highpass filter shown in FIG. 4. The differential input signals are V₂ -V₁ =100 sin 2πft mV, where F is frequency and t is time. The test frequency F was selected between 10 Hz and 1000 Hz, and the test results are represented by the symbol `o`. It is clear that the output voltage will decrease as the test frequency decreases. The units of the horizontal and vertical axex in FIG. 6 are Hertz (Hz) and dB, respectively. The frequency response of the implementation shown in FIG. 6 is concordant with that of the computer simulation shown in FIG. 2.

It can be readily understood from above that the proposed filters shown in FIGS. 1 and 4 have the functions of second-order highpass filtering and differential amplification.

The second-order differential highpass filter constructed in accordance with the present invention may be fabricated as a single chip IC or part of any other signal processing IC.

Having thus described the present invention, which is now deemed appropriate for Letters Patent is set out in the following appended claims. 

What is claimed is:
 1. A second-order differential highpass filter comprising a difference amplifier and a feedback processing circuit,said difference amplifier comprising an operational amplifier OP₁, and four resistors R₃, R₄, R₅ and R₆, wherein R₄ /R₃ =R₆ /R₅, the resistors R₅ and R₃ are connected to the inverting terminal (-) and the noninverting terminal (+) of the operational amplifier OP₁, respectively, so that an input voltage V₁ is fed to the inverting terminal (-) of the operational amplifier OP₁ via the resistor R₅, and another input voltage V₂ is fed to the noninverting terminal (+) of the operational amplifier OP₁ via the resistor R₃, and wherein the output of the operational amplifier OP₁ is fed back to the inverting terminal (-) of the operational amplifier OP₁ via the resistor R₆ ; and said feedback processing circuit comprising an operational amplifier OP₂, two resistors R₁ and R₂, and two serial capacitors C₂ and C₁ which are connected with each other in series with a conductive wire, wherein the output of the operational amplifier OP₂ is fed back to the inverting terminal (-) of the operational amplifier OP₂ via the two serial capacitors C₁ and C₂, and is connected to the noninverting terminal of the operational amplifier OP₁ via the resistor R₄, and the noninverting (+) terminal of the operational amplifier OP₂ is grounded, and wherein the inverting terminal of the operational amplifier OP₂ is connected to the output terminal of the operational amplifier OP₁ via the resistor R₁, and wherein one terminal of the resistor R₂ is connected to the conductive wire and another terminal thereof is grounded.
 2. A second-order differential highpass filter comprising an instrumentation amplifier and a feedback processing circuit,said feedback processing circuit comprising an operational amplifier OP₂, two resistors R₁ and R₂, and two serial capacitors C₂ and C₁ which are connected with each other in series with a conductive wire, wherein the output of the operational amplifier OP₂ is fed back to the inverting terminal (-) of the operational amplifier OP₂ via the two serial capacitors C₁ and C₂, and is connected to an reference terminal of the instrumentation amplifier, and the noninverting (+) terminal of the operational amplifier OP₂ is grounded, and wherein the inverting terminal of the operational amplifier OP₂ is connected to an output terminal of the instrumentation amplifier via the resistor R₁, and wherein one terminal of the resistor R₂ is connected to the conductive wire and another terminal thereof is grounded. 